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153.7.2 Trends in Global Mean Sea Level and Components
Tide gauges with the longest nearly continuous records of sea level show increasing sea level over the 20th century (Figure 3.12; Woodworth et al., 2009; Mitchum et al., 2010). There are, however, significant interannual and decadal-scale fluctuations about the average rate of sea level rise in all records. Different approaches have been used to compute the mean rate of 20th century global mean sea level (GMSL) rise from the available tide gauge data: computing average rates from only very long, nearly continuous records (Douglas, 2001; Holgate, 2007); using more numerous but shorter records and filters to separate nonlinear trends from decadal-scale quasi-periodic variability (Jevrejeva et al., 2006, 2008); neural network methods (Wenzel and Schroeter, 2010); computing regional sea level for specific basins then averaging (Jevrejeva et al., 2006, 2008; Merrifield et al., 2009; Wöp- pelmann et al., 2009); or projecting tide gauge records onto empirical orthogonal functions (EOFs) computed from modern altimetry (Church et al., 2004; Church and White, 2011; Ray and Douglas, 2011) or EOFs from ocean models (Llovel et al., 2009; Meyssignac et al., 2012). Different approaches show very similar long-term trends, but noticeably different interannual and decadal-scale variability (Figure 3.13a). Only the time series from Church and White (2011) extends to 2010, so it is used in the assessment of rates of sea level rise. The rate from 1901 to 2010 is 1.7 to 1.9 mm yr –1 (Table 3.1), which is unchanged from the value in AR4. Rates computed using alternative approaches over the longest common interval (1900–2003) agree with this estimate within the uncertainty. Since AR4, significant progress has been made in quantifying the uncertainty in GMSL associated with unknown VLM and uncertainty in GIA models. Differences between rates of GMSL rise computed with and without VLM from GPS are smaller than the estimated uncertainties (Merrifield et al., 2009; Wöppelmann et al., 2009). Use of different GIA models to correct tide gauge measurements results in differences less than 0.2 mm yr –1 (one standard error), and rates of GMSL rise computed from uncorrected tide gauges differ from rates computed from GIA-corrected gauges by only 0.4 mm yr –1 (Spada and Galassi, 2012), again within uncertainty estimates. This agreement gives increased confidence that the 20th century rate of GMSL rise is not biased high due to unmodeled VLM at the gauges. Satellite altimetry can resolve interannual fluctuations in GMSL better than tide gauge records because less temporal smoothing is required (Figure 3.13b). It is clear that deviations from the long-term trend can exist for periods of several years, especially during El Niño (e.g., 1997– 1998) and La Niña (e.g., 2011) events (Nerem et al., 1999; Boening et al., 2012; Cazenave et al., 2012). The rate of GMSL rise from 1993– 2010 is 3.2 to 3.6 mm yr –1 based on the average of altimeter time series published by multiple groups (Ablain et al., 2009; Beckley et al., 2010; Leuliette and Scharroo, 2010; Nerem et al., 2010; Church and White, 2011; Masters et al., 2012, Figure 3.13). As noted in AR4, this rate continues to be statistically higher than that for the 20th century. Since AR4, estimates of both the thermosteric component and mass component of GMSL rise have improved, although estimates of the mass component are possible only since the start of the GRACE meas- urements in 2002. After correcting for biases in older XBT data 3.2, the rate of thermosteric sea level rise in the upper 700 m since 1971 is 50% higher than estimates used for AR4 (Domingues et al., 2008; Wijffels et al., 2008). Because of much sparser upper ocean measurements before 1971, we estimate the trend only since then (Section 3.2). The warming of the upper 700 m from 1971 to 2010 caused an estimated mean thermosteric rate of rise of 0.6 to 0.8 mm yr –1 (90% confidence), which is 30% of the observed rate of GMSL rise for the same period (Table 3.1; Figure 3.13c). Although still a short record, more numerous, better distributed, and higher quality profile measurements from the Argo program are now being used to estimate the steric component for the upper 700 m as well as for the upper 2000 m (Domingues et al., 2008; Willis et al., 2008, 2010; Cazenave et al., 2009; Leuliette and Miller, 2009; Leuliette and Willis, 2011; Llovel et al., 2011; von Schuckmann and Le Traon, 2011; Levitus et al., 2012). However, these data have been shown to be best suited for global analyses after 2005 owing to a combination of interannual variability and large biases when using data before 2005 owing to sparser sampling (Leuliette and Miller, 2009; von Schuckmann and Le Traon, 2011). Comparison of sparse but accurate temperature measurements from the World Ocean Circulation Experiment in the 1990s with Argo data from 2006 to 2008 also indicates a significant rise in global ther- mosteric sea level, although the estimate is uncertain owing to relatively sparse 1990s sampling (Freeland and Gilbert, 2009). Observations of the contribution to sea level rise from warming below 700 m are still uncertain due to limited historical data, especially in the Southern Ocean (Section 3.2). Before Argo, they are based on 5-year averages to 2000 m depth (Levitus et al., 2012). From 1971 to 2010, the estimated trend for the contribution between 700 m and 2000 m is 0.1 to 0.2 mm yr –1 (Table 3.1; Levitus et al., 2012). To measure the contribution of warming below 2000 m, much sparser but very accurate temperature profiles along repeat hydrographic sections are utilized (Purkey and Johnson, 2010; Kouketsu et al., 2011). The studies have found a significant warming trend between 1000 and 4000 m within and south of the Sub-Antarctic Front (Figure 3.3). The estimated total contribution of warming below 2000 m to global mean sea level rise between about 1992 and 2005 is 0.1 to 0.2 mm yr –1 (95% confidence as reported by authors; Purkey and Johnson, 2010). Detection of the mass component of sea level from the GRACE mission was not assessed in AR4, as the record was too short and there was still considerable uncertainty in the measurements and corrections required. Considerable progress has been made since AR4, and the mass component of sea level measured by GRACE has been increas- ing at a rate between 1 and 2 mm yr –1 since 2002 (Willis et al., 2008, 2010; Cazenave et al., 2009; Leuliette and Miller, 2009; Chambers et al., 2010; Llovel et al., 2010; Leuliette and Willis, 2011). Differences between studies are due partially to the time periods used to compute trends, as there are significant interannual variations in the mass component of GMSL (Willis et al., 2008; Chambers et al., 2010; Llovel et al., 2010; Boening et al., 2012), but also to substantial differences in GIA corrections applied, of order 1 mm yr –1 . Recent evaluations of the GIA correction have found explanations for the difference (Chambers et al., 2010; Peltier et al., 2012), but uncertainty of 0.3 mm yr –1 is still probable. Measurements of sea level from altimetry and the sum of observed steric and mass components are also consistent at monthly scales during the time period when Argo data have global distribution (Figure 3.13d), which gives high confidence that the current ocean observing system is capable of resolving the rate of sea level rise and its components.